This invention relates to threaded parts for aircraft which are used under extremely large temperature fluctuations, and more particularly to threaded parts for aircraft made of heat-resistant steel.
Threaded parts such as nuts and bolts used for aircraft are required to stably and reliably maintain their function and quality even under conditions far severer than normally expected use conditions to ensure safety of aircraft. This requirement is especially severer for threaded parts used in portions associated with aircraft engines and fuselages because the failure of such threaded parts can directly lead to a grave accident and also threaded parts used in such portions are subjected to especially large temperature fluctuations.
Threaded parts used at such portions are typically exposed to extremely cold air at high altitude, cold air from cooled portions of the aircraft or low temperatures conducted therefrom, and hot air from engines or heat conducted therefrom. Thus, resistance to thermal stress is also required for such threaded parts.
Typical materials for such threaded parts for aircraft include alloy steel, corrosion-resistant steel and heat-resistant steel SUH660 (A-286, JIS G 4311). Heat-resistant steel (SUH660) is an austenite Fe--Ni--Cr alloy of a precipitation-hardening type, and exhibits high strength and corrosion resistance up to 704.degree. C. In the aerospace field, its maximum applicable temperature is said to be 649.degree. C. (1200.degree. F.). Thus, it is usually strengthened by aging treatment at 700-780.degree. C.
Among threaded parts for aircraft, nuts and bolts used at or near combustion chambers of the engine tend to be subjected to severe thermal stresses. That is, these threaded parts tend to be partially or entirely subjected to thermal shocks due to temperature fluctuations ranging from about 600.degree. C. to about 0.degree. C. Safety consideration is therefore given to these parts so as not to be cracked or destroyed due to such thermal shocks.
Compared with alloy steel, threaded parts for aircraft made of heat-resistant steel are physically characterized by the facts that thermal conductivity is about 1/3, that thermal expansion coefficient is about 1.5 times higher, and that friction coefficient is high. Due to synergistic effect of these characteristics, threaded parts tend to seize (or stick fast through excessive heat and get locked and unturnable) when tightened by power tools or hand wrenches. Thus, to prevent seizure, conventional such threaded parts are electrically plated with cadmium or nickel, or have their threaded surface coated with a resin containing a lubricant.
Another problem is that when subjected to large temperature fluctuations and vibrations for a long period of time, their plated or resin coated layers tend to crack due to thermal stresses resulting from a difference in thermal expansion coefficient between the plated layer or resin coated layer and the heat-resistant steel substrate.
If subjected to thermal stresses repeatedly, these layers may peel off. Once outer layers peel off, two threaded parts meshing each other would be locally strained due to thermal expansion and eventually seize.
Once seized, threaded parts cannot be tightened any further nor loosened. Thus, removing seized threaded parts requires a lot of trouble. That is, they have to be cut. This may make maintenance of aircraft parts impossible.
An object of this invention is to provide threaded parts for aircraft made of heat-resistant steel which will never seize even after subjected to large temperature fluctuations and vibrations for a long time, and which can be easily tightened and loosened for maintenance and inspection.